The present invention relates to swinging bucket rotors for centrifuges, and is directed more particularly to a swinging bucket rotor having an improved bucket seating arrangement.
Under circumstances in which relatively large sample vessels or relatively large numbers of small sample vessels are to be centrifuged, it is a common practice to use centrifuges having swinging bucket rotors. Rotors of the latter type include a plurality of symmetrically located openings in which a plurality of buckets are hung by respective pairs of bucket pins. When the rotor is stationary, these buckets assume a substantially vertical position for easy loading. As the rotor is accelerated, however, the bottoms of the buckets swing outwardly until they come into contact with a seating surface on the periphery of the rotor. This seating surface provides radial support to the buckets and thereby prevents large radial loading forces from being applied to the bucket pins.
In rotors of the above type, the bucket pins are ordinarily mounted in spring-loaded carrier assemblies. When the rotor is stationary, or is rotating only slowly, these assemblies serve to hold the buckets close enough to the center of the rotor that they can swing freely without contacting the seating surface. When the rotor is rotating at a high speed, however, these assemblies allow the buckets to move outwardly from the center of the rotor and to seat on their respective seating surfaces. Thus, the spring-loaded carrier assemblies assure that the buckets pivot freely and yet seat securely.
One important objective in the design of a swinging bucket rotor is that each bucket seat in a position in which little or no side loading forces act on its bucket pins, i.e., no forces which act in a direction parallel to the rotational axis of the rotor. This, in turn, makes it desirable to assure that, as each bucket seats, its pins tend to lift slightly off of the surfaces of the respective carrier assemblies. This lift-off is desirable because it assures that the pins of the bucket are not in contact with any surface that is capable of generating side loading forces.
One serious problem with swinging bucket rotors is that the desired seating condition may not be established if the buckets are loaded unevenly, i.e., so that they do not rest vertically when the rotor is stationary. This is because the uneven loading of a bucket changes the angle at which the bottom of that bucket first comes into contact with the seating surface. If, for example, the angle is such that the outer or leading edge of the bucket contacts the seating surface before the inner or trailing edge thereof, the friction between the leading edge and the seating surface can prevent the bucket from swinging into the desired seated position. If this occurs, the continued acceleration of the rotor will produce a moment which generates substantial side loading forces in the bucket pins. While these side loading forces may not be sufficient to cause the bucket pin to fail immediately, they can be sufficient to produce fatigue which will eventually cause the bucket pin to fail.
Prior to the present invention, the above-described seating problem was dealt with by locating the bucket pins above the center line of the bucket. This off-center location tended to assure that the trailing edge of the bucket contacted the seating surface prior to the leading edge thereof. The latter contact sequence, in turn, assured that, as the bucket seated, it tended to rotate slightly in a direction which caused the bucket pins to lift off of their carrier assemblies and thereby be relieved of any side loading forces.
While the use of off-center bucket pins tended to solve the bucket seating problem, it also introduced a non-symmetry into the structure of the bucket. This non-symmetry not only increased the cost of the bucket, but also created the risk that an inattentive operator could insert the bucket with a reversed orientation, i.e., with an orientation 180.degree. from its intended orientation. If such a reversal did occur, it tended to aggravate rather than lessen the original seating problem. Under such conditions, the moment developed by the reversed bucket could cause extremely large side loading forces to develop in the bucket pins and thereby initiate a catastrophic failure. Thus, the solution which the prior art provided for the bucket seating problem was both costly and potentially dangerous.
The existing art contemplates a sloped seating surface, i.e., one which is slightly inclined with respect to the axis of the rotor. The existing art contemplates the use of carrier assembly springs which have a stiffness that allows the leading edges of the bucket bottoms to clear the seating surfaces before the trailing leading edges thereof come into contact therewith. As the buckets seat, they rotate in directions which raise their bucket pins slightly off of the surfaces of the respective carrier assemblies and thereby prevent side loading forces from being applied thereto.